Support for Single-Shard multi document ACID transactions

To support benefits of ACID transactions with fully-denormalized document data modeling, MongoDB added single-shard ACID transaction support in the released 4.0. These single-shard transactions apply ACID properties on updates across multiple documents those are present in the same shard. Multi-shard transactions supposed to implement in release 4.2.

Scenario, how it works?

1. Client application will get a database session

2. Database session initiate transaction block using start_transaction statement with majority writeConcern in try block. The only writeConcern thats suitable for high data durability is that of majority. This means a majority of replicas should commit the changes before the primary acknowledges the success of the write to the client. The transaction will remain blocked till at least 1 of the 2 secondaries pulls the update from the primary using asynchronous replication which is susceptible to unpredictable replication lag especially under heavy load.

3. Within transaction block multiple insert/update/remove statements can be made.

4. At end of transaction block commit_transaction required to commit data in database.
5. If any exception occurs in transaction block, client application should abort_transaction in catch block to roll back updates.
6. Finally the database client session can be closed in finally block

Sample Java code to mimic same scenario,

###Java sample
private void multiDocumentTransactions() {
    ClientSession session = client.startSession();
    try {
        session.startTransaction(TransactionOptions.builder().writeConcern(WriteConcern.MAJORITY).build());
        orders.updateOne(session, {..filter..}, {..updates..});
        stock.updateOne(session, {..filter..}, {..updates..});
        session.commitTransaction();
    } catch (MongoCommandException e) {
        session.abortTransaction();
        //ROLLBACK TRANSACTION
    } finally {
        session.close();
    }
}